The real answer is that there isn't always any way to tell if an image is AI generated, and as time passes while there may always be tells for poorly done images the percentage of generated images displaying those tells will continue to rapidly shrink.
You can only find signs an image was AI generated in images that display those signs. That doesn't mean you're weeding them all out, or even 1% of them.
Even if you spend a month looking at every tiny detail of an image, unless you physically witnessed it's creation or it's something with known provenance (i.e. the Mona Lisa) you can never say for certain that it is not AI generated.